Swift Observations of GRB 210726A

M.G. Bernardini (INAF-OAB) and A. D'Ai (INAF-IASFPA) for the Swift team

1. Introduction

At 19:19:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210726A (trigger=1061687) (Bernardini et al. GCN Circ. 30523). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 63° from the Sun (4.5 hours East) and 150° from the 91%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Palmer et al. (GCN Circ. 30536), the BAT ground-calculated position is RA, Dec = 193.333, 19.211 deg which is RA(J2000) = 12h53m19.9s Dec(J2000) = +19°12'38.9" with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a short spike that starts at ~T0, peaks at ~T+0.2 s, and ends at ~T+0.4 s. Some tail emission from the raw light curves was mentioned in the original circular (Bernardini et al., GCN Circ. 30523), and this potential weak tail can be seen in the 1-s binned light curve (15-350 keV) as well. However, image analyses from ~T+0.5 s to ~T+2 s do not find any significant detections that are >~ 2 sigma. Therefore, this signal is also consistent with noise fluctuation. T90 (15-350 keV) is 0.39 ± 0.11 s (estimated error including systematics).

The time-averaged spectrum from T-0.00 to T+0.42 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.54 ± 0.47. The fluence in the 15-150 keV band is 4.5 ± 1.1 x 10-8 erg cm-2. This fluence is larger than that of 39% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.29 s in the 15-150 keV band is 0.7 ± 0.3 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1061687/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 30537). We have analysed 42 ks of XRT data for GRB 210726A, from 79 s to 839.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 30524).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.70 (+0.10, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 ± 0.3. The best-fitting absorption column is 3.3 (+1.3, -1.2) x 1021 cm-2, in excess of the Galactic value of 1.7 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (5.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.3 (+1.3, -1.2) x 1021 cm-2
Galactic foreground: 1.7 x 1020 cm-2
Excess significance: 4.5 σ
Photon index: 2.2 ± 0.3

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01061687.

4. UVOT Observations and Analysis

UVOT results are not available.

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s-1 illuminated-detector-1. The vertical green dash-dotted lines show the T50 interval, the vertical black dashed lines show the T90 interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
12h53m09.80s +19°11'24.7" 2.1" XRT-final UKSSDC
12h53m09.82s +19°11'25.1" 2.4" XRT-enhanced Osborne et al. GCN Circ. 30524
12h53m19.9s +19°12'38.9" 2.6' BAT-refined Palmer et al. GCN Circ. 30536

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30522 Swift GRB210726.80: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 30525 CAHA 2.2m telescope optical limit CAHA upper limits
Optical Watson et al. 30534 RATIR Optical Upper Limits and
Candidate Host Galaxy 		RATIR upper limits
Optical Kann et al. 30543 Deep CAHA 2.2m limit CAHA upper limits
Optical Kann et al. 30545 OSIRIS/GTC observations GTC
Radio Schroeder et al. 30542 6 GHz VLA radio upper limit VLA upper limits
Radio Schroeder et al. 30658 VLA radio afterglow detection VLA detection
X-ray Escorial et al. 30558 Chandra detection of the X-ray
afterglow 		Chandra detection
Gamma-ray Veres 30540 Fermi GBM Sub-Threshold Detection Fermi GBM Epeak=70 keV
Other 30535 Likely a short burst

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

August 19, 2021